1H and 13C NMR studies of glycine in anisotropic media: double-quantum transitions and the effects of chiral interactions

The (1)H NMR spectrum of glycine in stretched gelatin gel and in cromolyn liquid crystal displays a well-resolved doublet due to (1)H-(1)H dipolar interaction. Multiple spectra were obtained within a wide range of offset frequencies of partially saturating radio-frequency (RF) radiation to generate steady-state irradiation envelopes or z-spectra of glycine. Maximal suppression of the doublet occurred when the irradiation was applied exactly at the centre frequency, between the two glycine peaks. This phenomenon is due to double-quantum transitions and is similar to our previous work on quadrupolar nuclei (2)H (HDO) and (23)Na(+). When the (13)C isotopomer glycine-2-(13)C was used, the same effect was found in twice, split by (1)J(CH)+2D(CH). Additional signals in (1)H and (13)C NMR due to prochiral-chiral interactions were found when glycine-2-(13)C was dissolved in chiral anisotropic gelatin and κ-carrageenan gels. The NMR spectra were successfully simulated assuming a (2)J(HH) coupling constant of -16.5Hz and two distinct dipolar coupling constants for the -(13)CH(2)- group (D(C,HA), and D(C,HB)).     

Double quantum transition as the origin of the central dip in the z-spectrum of HDO in variably stretched gel

The ²H NMR spectrum of HDO in gelatin gel, that is stretched inside silicone rubber tubing, displays a well resolved doublet. Spectra were obtained with a range of offset frequencies of partially saturating radio-frequency (RF) radiation. The resulting steady-state irradiation envelope (also referred to as a ‘z-spectrum’) has the peculiar feature that maximal suppression of the doublet occurs when the irradiation is applied exactly at the centre frequency, between the two HDO peaks. We present a quantum mechanical explanation for this phenomenon. It is shown that the phenomenon is the result of double quantum transitions. The analysis is extendable to more complex quadrupolar and dipolar-coupled systems of other nuclides. It has implications for enhancement of contrast in magnetic resonance imaging of heterogeneous systems using dipolar and quadrupolar interactions.     

Isotope Effects on the 17O, 1H Coupling Constant and the 17O-{1H} Nuclear Overhauser Effect in Water

The NMR spectra of solutions of 30% 17O-enriched H2O and D2O in nitromethane display the resonances of the three isotopomers H2O, HDO, and D2O. All 17O, 1H and 17O, 2H coupling constants and the primary and secondary isotope effects on J(17O, 1H) have been determined. The primary effect is -1.0 +/- 0.2 Hz and the secondary effect is -0.07 +/- 0.04 Hz. Using integrated intensities in the 17O NMR spectra, the equilibrium constant for the reaction H2O + D2O right harpoon over left harpoon 2HDO is found to be 3.68 +/- 0.2 at 343 K. From the relative integrated intensities of proton-coupled and -decoupled spectra the 17O-{1H} NOE is estimated for the first time, resulting in values of 0.908 and 0.945 for H2O and HDO, respectively. This means that dipole-dipole interactions contribute about 2.5% to the overall 17O relaxation rate in H2O dissolved in nitromethane. Copyright 1999 Academic Press.     

Determination of membrane Peptide orientation by 1H-detected 2H NMR spectroscopy

We demonstrate the application of the proton inverse detected deuteron (PRIDE) NMR technique to the measurement of the orientation of membrane-bound peptides with enhanced sensitivity. Gramicidin D, a transmembrane peptide, and ovispirin, a surface-bound peptide, were used as model systems. The peptides were 2H-labeled by 1H/2H exchange and oriented uniaxially on glass plates. The directly detected 2H spectra of both peptides showed only a strong D(2)O signal and no large quadrupolar splittings. In contrast, the PRIDE spectrum of gramicidin exhibited quadrupolar splittings as large as 281 kHz, consistent with its transmembrane orientation. Moreover, the large D(2)O signal in the directly detected 2H spectra was cleanly suppressed in the PRIDE spectrum. For ovispirin, the 1H indirectly detected 2H spectrum revealed a 104 kHz splitting and a zero-frequency peak. The former reflects the in-plane orientation of most of the helix axis, while the latter results from residues with a magic-angle orientation of the N-D bonds. These are consistent with previous 15N NMR results on ovispirin. The combination of PRIDE and exchange labeling provides an economical and sensitive method of studying membrane peptide orientations in lipid bilayers without the influence of D(2)O and with the ability to detect N-D bonds at the magic angle from the bilayer normal.     

Influence of local molecular motions on the determination of 1H-1H internuclear distances measured by 2D 1H spin-exchange experiments

Analysis of spin-exchange build-up curves obtained by measurement of 2D 1H CRAMPS spectra of alpha-glycine was performed to evaluate the rate of 1H-1H spin-exchange process with respect to the influence of variation in internal molecular motion. Differences in local motions significantly affect spin-exchange constants even in highly rigid organic solids with virtually uniform motion behavior. The polarization transfer between nonequivalent alpha-protons is described by the spin-exchange constant D=0.77 nm(2)ms(-1), while the polarization transfer involving spin exchange between alphaH and NH(3)(+) protons is characterized by D=0.24-0.21 nm(2)ms(-1). This significant decrease corresponds to rotation of hydrogen-bonded amino groups. Neglecting this variation in local spin-exchange constants the resulting calculated 1H-1H distance can be overestimated by up to 100%. Complications following from relayed and back polarization transfer involving the nearest spins within one functional group (e.g., CH(2) and/or NH(3)(+)) and intermolecular spin exchange are discussed. It was shown that 2H quadrupolar splitting determined for selected sites directly correlates with the experimentally observed differences in spin-exchange coefficients. It is also demonstrated that a medium level quantum chemical calculation of molecular dynamics provides relevant data that can be used to estimate differences in molecular motions.     

用二维NMR交换谱定量地研究氘代DMSO和氘代氯仿中小肽活泼质子的交换动力学

用核磁共振二维交换谱方法研究了类肽化合物Cbz-Ser-Tyr-OC₆H₁₃中活泼质子的交换动力学,结果表明,在DMSO-d₆和CDCl₃溶液中,四种活泼质子与溶剂中残留水的交换快慢顺序为:Ser-OH > Tyr-OH > Ser-NH > Tyr-NH.在此基础上,对氨基酸中氨基所处的局部环境进行了讨论.     

Dynamic NMR Study and Detailed 1H and 13C Assignments of an ADAR Precursor in Solvent

A nitrogen-containing compound, named methyl-4-phenyl-2-(2,4,6-triisopropyl-phenylsulfonamido)but-3-enoate (MPSN), is used as a precursor for inverse-electron-demand aza-Diels–Alder reaction. Its ¹H and ¹³C nuclear magnetic resonances (NMR) were fully assigned by a combination of one- and two-dimensional NMR experiments. It was found that this compound had two conformers (a and b) in the process of assignments. Exchange spectroscopy (EXSY) was used to study the exchange dynamics of this compound in solvent. On the basis of ¹H NMR and EXSY experiments at various temperatures, thermodynamic and dynamic parameters of the conformational exchange process were deduced for this compound. The free-energy activation (ΔG ^≠) at 298 K is 15.07 ± 0.6 kcal mol⁻¹, which suggests that the conformational exchange might be the result of C=N bond rotation.     

Quantitative analysis of 17O exchange and T1 relaxation data: application to zirconium tungstate

The theoretical basis behind a recent quantitative analysis of 17O exchange in ZrW2O8 [M.R. Hampson, J.S.O. Evans, P. Hodgkinson, J. Am. Chem. Soc. 127 (2005) 15175-15181] is set out. Despite the complexities of combining the multi-exponential relaxation of half-integer quadrupolar nuclei with chemical exchange, it is shown how magnetisation transfer experiments can be analysed to obtain estimates of absolute exchange rates. The multi-exponential relaxation is best modelled using a magnetic mechanism, i.e. the rapid T1 relaxation observed, particularly at high temperatures, can be directly related to the relatively high degree of 17O labelling employed. The combination of the 1D EXSY results with T1 values as a function of temperature provides exchange rates and activation barriers over a wide temperature range (40-226 degrees C).     

2D EXSY实验和几何平均法研究水与天然化合物的羟基之间的化学交换速率常数

利用二维核磁共振交换谱(2DEXSY)研究了三种天然产物的羟基质子与水的交换,三种天然产物分别含有四到八个羟基质子.较为简单的几何平均法(GA)被用来进行数据处理.GA法在假定体系和实际体系中,与全矩阵分析和初级速率近似比较,结果尚好。     

Calculation of pulsed NMR signal in I = 3/2 quadrupolar spin system

The rf pulse response of I=3/2 spin system experiencing first order quadrupolar splitting is studied using density matrix approach. A general expression is derived in terms of spin populations, quadrupole splitting and duration and amplitude of the rf pulse for calculating the NMR signal arising due to the centre line and satellite resonances for the situation where the impressed rf pulse excites the resonances selectively as well as non-selectively. The necessary 4×4 transformation matrix obtained analytically by diagonalyzing the Hamiltonian are used to get the expression for the centre line response. The satellite signals are obtained in the same way but by using the numerical values of the roots of the related quartics. The widths of the corresponding π/2-pulses are calculated for different initial spin populations. The variations of this pulse-width and the corresponding signal amplitude as a function of satellite splitting are studied.     

Calculation of pulsed NMR signal in I=3/2 quadrupolar spin system

The rf pulse response of I=3/2 spin system experiencing first order quadrupolar splitting is studied using density matrix approach. A general expression is derived in terms of spin populations, quadrupole splitting and duration and amplitude of the rf pulse for calculating the NMR signal arising due to the centre line and satellite resonances for the situation where the impressed rf pulse excites the resonances selectively as well as non-selectively. The necessary 4×4 transformation matrix obtained analytically by diagonalyzing the Hamiltonian are used to get the expression for the centre line response. The satellite signals are obtained in the same way but by using the numerical values of the roots of the related quartics. The widths of the corresponding π/2-pulses are calculated for different initial spin populations. The variations of this pulse-width and the corresponding signal amplitude as a function of satellite splitting are studied.     

Pure-phase two-dimensional niobium-93 nutation spectroscopic study of lead metaniobate and the piezoelectric lead magnesium niobate

An NMR technique to measure pure-phase two-dimensional nutation NMR spectra, that yields higher resolution than traditional nutation experiments is reported. Using this technique 93Nb nutation NMR spectra of PbNb2O6 and the technologically important Pb(Mg1/3Nb2/3)O3 (PMN) have been measured and the quadrupolar coupling constant of the niobium site in PbNb2O6 (C(Q) = 19 +/- 2 MHz) determined. Estimates of the quadrupolar coupling constants for three different resonances associated with different niobium(V) sites in PMN (C(Q) < 1.2 MHz, approximately 17 MHz, and > 62 MHz) are also reported.     

1H NMR studies on reduced high potential iron protein (HIPIP) from Chromatium Vinosum

¹H NMR studies were performed on reduced high potential iron protein from Chromatium Vinosum, focusing on the assignment of the resonances of the aromatic residues. Despite the residual paramagnetism of the iron sulfur cluster, the complete ring proton spin system and sequence specific assignment of all tryptophan residues (Trp⁸⁰, Trp⁷⁶, Trp⁶⁰) was achieved. Tentative assignments are proposed for Tyr¹⁹ and one Phe residue. Implications of the above assignments are discussed. 2D EXSY experiments permitted correlation of the resonances in the reduced and the oxidized form. Similar NOESY pattern for the indole NH’s were observed, suggesting substantial retention of the gross structural features upon oxidation.     

Isotope Effects on theO,H Coupling Constant and theO–{H} Nuclear Overhauser Effect in Water

The NMR spectra of solutions of 30%¹⁷O-enriched H₂O and D₂O in nitromethane display the resonances of the three isotopomers H₂O, HDO, and D₂O. All¹⁷O,¹H and¹⁷O,²H coupling constants and the primary and secondary isotope effects onJ(¹⁷O,¹H) have been determined. The primary effect is −1.0 ± 0.2 Hz and the secondary effect is −0.07 ± 0.04 Hz. Using integrated intensities in the¹⁷O NMR spectra, the equilibrium constant for the reaction H₂O + D₂O 2HDO is found to be 3.68 ± 0.2 at 343 K. From the relative integrated intensities of proton-coupled and -decoupled spectra the¹⁷O–{¹H} NOE is estimated for the first time, resulting in values of 0.908 and 0.945 for H₂O and HDO, respectively. This means that dipole–dipole interactions contribute about 2.5% to the overall¹⁷O relaxation rate in H₂O dissolved in nitromethane.     

一维NOE NMR 技术用于蒂巴因衍生物的立体结构研究

基于高效镇痛剂双氢埃托啡游离碱,对蒂巴因衍生物的立体化学结构的NMR 测定方法进行了研究．结果表明,采用多频率位移选择激发一维NOESY(1D NOESY)和二维NMR(2D NMR)相结合的方法,可有效解决该类化合物¹H 和¹³C NMR 谱复杂重叠的信号归属困难的问题．同时在阐明溶液构象的基础上,提出采用一维NOE(1D NOE)测定C-7和差向异构体的新方法．该方法简单、有效、不受样品浓度影响和杂质信号干扰,结果可靠,不需要标准品对照等优点．该结果有助于此类化合物的立体结构与生物活性关系的研究．     

Non-bridging oxygens in borate glasses: characterization by 11B and 17O MAS and 3QMAS NMR

The concentrations of non-bridging oxygens (NBO) in oxide glasses has major effects on their properties and on those of their precursor glass melts. In borate and borosilicate glasses, the presence of NBO bonded to boron has generally been inferred from 11B NMR spectra and mass balance considerations. Here we report the direct observation of such NBO using 17O MAS and 3QMAS techniques, and compare estimates of their populations with those derived from high-resolution 11B MAS spectra. For the latter, two independent methods are used, based on the ratios of trigonal to tetrahedral boron and on the concentrations of trigonal boron sites with large quadrupolar asymmetry parameters. We include data on crystalline sodium pyroborate (Na4B2O5) and sodium metaborate (NaBO2), and several sodium and barium borate glasses. 17O chemical shifts and quadrupolar coupling constants for NBO bonded to boron vary considerably depending on their coordination environment. In borosilicates, peaks for this species may be hidden by overlap with B-O-Si or Si-O-Si resonances.     

Water-filled MCM-41 characterized by double-quantum-filtered2H NMR spectral analysis

The dynamics of water molecules confined in adsorbed layers of siliceous MCM-41 with a pore diameter of 2.8 nm is investigated at 230 K by deuteron nuclear magnetic resonance (NMR) relaxation studies including line shapes of theT ₁ process and double quantum filtered (DQF) spectral analyses.²H DQF NMR is a particularly sensitive tool for the determination of the adsorbate dynamics resulting from residual quadrupolar interaction due to the local order. The amount of monolayer water is determined. The monolayer water is composed of two different water components characterized by water, with isotropic reorientational motions, exchanging with water displaying a solid-like spectrum with 4 kHz edge splitting. One may expect that the latter water is situated on surface sites in MCM-41. The restricted wobbling motion of the D-O bond is used to describe its dynamics which is one order of magnitude slower than the isotropic reorientational motion. The order parameter, the motional correlation time, and the exchange rate thus determined provide useful information on the structure and the adsorptive properties of the mesoporous system.     

2H NMR and polyelectrolyte-induced domains in lipid bilayers

2H NMR studies of polyelectrolyte-induced domain formation in lipid bilayer membranes are reviewed. The 2H NMR spectrum of choline-deuterated phosphatidylcholine (PC) reports on any and all sources of lipid bilayer surface charge, since these produce a conformation change in the choline head group of PC, manifest as a change in the 2H NMR quadrupolar splitting. In addition, homogeneous and inhomogeneous surface charge distributions are differentiated. Adding polyelectrolytes to lipid bilayers consisting of mixtures of oppositely charged and zwitterionic lipids produces 2H NMR spectra which are superpositions of two Pake sub-spectra: one corresponding to a polyelectrolyte-bound lipid population and the other to a polyelectrolyte-free lipid population. Quantitative analysis of the quadrupolar splittings and spectral intensities of the two sub-spectra indicate that the polyelectrolyte-bound populations is enriched with oppositely charged lipid, while the polyelectrolyte-free lipid population is correspondingly depleted. The same domain-segregation effect is produced whether cationic polyelectrolytes are added to anionic lipid bilayers or anionic polyelectrolytes are added to cationic lipid bilayers. The 2H NMR spectra permit a complete characterization of domain composition and size. The anion:cation ratio within the domains is always stoichiometric, as expected for a process driven by Coulombic interactions. The zwitterionic lipid content of the domains is always statistical, reflecting the systems tendency to minimize the entropic cost of demixing charged lipids into domains. Domain formation is observed even with rather short polyelectrolytes, suggesting that individual polyelectrolyte chains aggregate at the surface to form "superdomains". Overall, the polyelectrolyte bound at the lipid bilayer surface appears to lie flat along the surface and to be essentially immobilized through its multiple electrostatic contacts.     

A simple theoretical treatment of quadrupolar effects on magic-angle-spinning solid-state NMR spectra of spin-1/2 nuclei in the limit of large quadrupole coupling constants

A simple approach is discussed for studying the effect of quadrupolar nuclei on the magic-angle-spinning solid-state NMR lines of spin-1/2 nuclei in the limit of large quadrupole coupling constants. Equations are derived both for the isotropic shifts and the Pake-like powder patterns for any quadrupolar spin and for arbitrary orientations of the internuclear vector with respect to the unique axis of an axially symmetric quadrupole tensor. First-order effects due to a small Zeeman perturbation on these lines are explored, as well as deviations from axial symmetry in the electric field gradient when S = 3/2 quadrupolar nuclei are involved. Spectral parameters likely to be observed in the case of coupling between ³¹P and ²⁰¹Hg are also discussed.     

N—甲基异靛蓝在不同溶剂中的^1H—NMR研究

用选择照射方法研究了N-甲基异靛蓝(isoindigatin)在(CD_3)_2CO、CDCl_3、DMSO-d_6溶剂中及重水交换后的~1H NMR谱。得到了化学环境极为类似的两芳环质子在不同溶剂中的NMR谱线归属和偶合常数。